1. Field of Invention
At least one embodiment of the invention relates to a polyphase power converter, and in particular, to a power factor corrected polyphase power converter.
2. Discussion of Related Art
Polyphase power converters may be employed to convert a multiphase AC input to a DC output. The DC output is often supplied to a DC bus or link. For example, a polyphase uninterruptible power supply (“UPS”) may include a polyphase power converter. The DC bus may connect the output of the polyphase power converter to an input of an inverter which is also included in the UPS. The inverter converts the DC power supplied by the DC bus to a polyphase AC signal at an output of the UPS. The UPS may also include a backup source of DC power (e.g., a battery power source, a DC generator, etc.). As is well known by those of ordinary skill in the art, the output of the UPS can be connected to electrical load to increase the reliability of the power supplied to the load.
Various types of UPS systems may employ polyphase power converters. For example, polyphase power converters may be employed in an on-line UPS that can supply power derived from either or both of a primary source of power and a backup source of power without interruption provided that at least one of the primary power source and the secondary power source is available. Polyphase power converters may also be employed in an off-line UPS system that includes a transfer switch such that there is an interruption in power supplied to the electrical load when primary power is lost unexpectedly.
Polyphase power converters may also be employed to convert multiphase AC power to DC power which is supplied to equipment that operates on DC power. For example, a polyphase power converter may supply DC power to operate DC motors or telecommunications equipment. The polyphase power converter may be integrated into the DC powered equipment, or the converter may be a stand-alone unit with an output connected to the DC equipment.
Such UPSs typically are connected to a polyphase AC input that includes a neutral. Generally, the UPS includes a continuous neutral connection from the UPS input to the UPS output. In many of these known approaches, the batteries that are employed with the UPS are connected to the neutral.
Various polyphase power converters include power factor control to maintain a unity power factor of the power drawn from the AC source. Known power converter topology often includes a number of circuits (e.g., boost circuits) that convert the AC power supplied to the input of the power converter to either a positive DC signal or a negative DC signal. In general, each phase of the polyphase AC source is connected to a single positive boost circuit and a single negative boost circuit. Typically, operation of the boost converters is controlled by pulse width modulation. For example, where the polyphase system is a 3-phase system, each phase is connected both to a positive boost circuit that supplies DC power to the positive DC output and to a negative boost circuit that supplies DC power to the negative DC output. FIG. 1 is a high-level schematic of one such polyphase power converter.
The system of FIG. 1 includes a polyphase AC source 100 (e.g., a utility power source) and a DC source 101 (e.g., a battery power source). The system also includes a power converter 102 which may, for example, be included in a UPS. The power converter 102 of FIG. 1 includes an AC input 106 that includes inputs 106A, 106B, 106C each connected to a phase of the polyphase AC source 100. The AC source 100 may also include a neutral which can be connected to a neutral 112 of the power converter 102 at the AC input 106. The power converter 102 of FIG. 1 also includes a DC input 107 including a positive DC input 109 and a negative DC input 111. In addition, the power converter 102 includes a DC output 115. In the embodiment shown in FIG. 1, the positive DC input 109 is connected to the DC source 101 at a positive DC terminal and the negative DC input 111 is connected to the DC source 101 at a negative DC terminal.
The power converter 102 includes a plurality of circuits 115A, 115B, 117A, 117B, 119A, 119B that convert the AC power supplied by the AC source 100 to DC power which is supplied to either a positive DC bus 108 or a negative DC bus 110 at the DC output 115. When employed in a UPS, each of the positive DC bus 108, the negative DC bus 110, and the neutral 112 can be supplied to further UPS circuitry (e.g., to an inverter) that converts the DC to an AC output voltage at the output of the UPS. Circuitry used to convert the DC to AC is well known to those of skill in the art and is not shown in FIG. 1.
Each phase of the polyphase AC source 100 (e.g., lines L1, L2, L3) is supplied to a positive boost circuit (e.g., boost circuits 115A, 117A, and 119A, respectively) and a negative boost circuit (e.g., boost circuits 115B, 117B, and 119B, respectively). Each boost circuit associated with lines L2, L3 (e.g., boost circuits 117A, 117B, and 119A, 119B, respectively) is substantially identical to those associated with line L1; therefore, a description of the operation of only boost circuits 115A and 115B is provided here. In the embodiment shown in FIG. 1, each of the plurality of circuits 115A, 115B, 117A, 117B, 119A, 119B include a rectifier (e.g., rectifiers 103A, 103B), a capacitor (e.g., capacitors 104A, 104B), a silicon controlled rectifiers (“SCR”) (e.g., SCRs 105A, 105B), an inductor (e.g., inductors 116A, 116B), a switching device (e.g., switching devices 118A, 118B), and a diode (e.g., diodes 120A, 120B).
Operation of the circuitry is well-known to those skilled in the art and is described in greater detail, for example, in International Application No. PCT/DK02/00041, filed on Jan. 22, 2002, by American Power Conversion Denmark APS, the disclosure of which is incorporated herein by reference.
Briefly, the two boost converter circuits (e.g., circuits 115A, 115B) are connected to a line (e.g., line L1) to operate during the positive half-cycle of the AC input and the negative half-cycle of the AC input, respectively, of the line to which they are connected. The positive boost converter circuit includes a line input 122A, a DC input 124A and a DC output 126A. The rectifier 103A (e.g., a diode) provides half wave rectification of the voltage and current supplied from the line L1 and conducts current during the positive half-cycles of the AC source. The capacitor 104A may be employed as a filter capacitor to eliminate electrical noise that would otherwise be transmitted from the polyphase power converter to the AC source 100. The inductor 116A is switchably connected to the neutral 112 by the switch 118A to store energy in the inductor during a first period of an operating cycle. In a second period of the operating cycle, the inductor 116A is disconnected from the neutral 112 when the switch 118A is turned off. When the inductor 116A is disconnected from the neutral 112, the energy stored in the inductor is provided to the positive DC bus 108 via a diode 120A. During the period when the inductor 116A is providing energy to the positive DC bus 108, a capacitor 122 is also charged.
During the negative half cycles of the line L1, boost circuit 115B which includes the rectifier 103B operates in a fashion similar to that described for the circuit 115A to provide power to the negative DC bus 110. The rectifier 103B provides half wave rectification of the voltage and current supplied from the line L1 and conducts current during the negative half cycles of the line L1. Each of the remaining boost circuits operate in a similar manner to supply power to the positive DC bus 108 and the negative DC bus 110 during the respective positive and negative half-cycles of each line, for example, where boost circuits 117A and 119A supply power to the positive DC bus 108, and boost circuits 117B and 119B supply power to the negative DC bus 110. Operation of the switches that provide the switching in the boost circuits is provided by control logic that, in general, switches the switches on and off in response to a comparison between a desired input current waveform and the existing input current waveform. Typically, operation of the boost converters is controlled by pulse width modulation. Further, the boost circuits may include power factor control to maintain a unity power factor of the power drawn from the AC source 100.
Power from the DC power source 101 can be supplied to the boost circuits 115A, 115B, 117A, 117B, 119A, 119B either alone or in combination with AC power from the AC source 100. Each boost circuit is connected to the DC source at a DC input (e.g., DC inputs 124A, 124B). Where the UPS includes batteries, for example, a first battery can be configured with a negative battery potential connected to the neutral 112 and a positive battery potential connected to the DC input 124A and from there to the remainder of the boost converter 115A via the silicon controlled rectifier (“SCR”) 105A. A second battery can be configured with a positive battery potential connected to the neutral 112 and a negative battery potential connected to the DC input 124B and from there to the remainder of the boost converter 115B via the SCR 105B. As a result, the source of DC power is switchably connected to the boost circuits (e.g., boost circuits 115A, 115B).
The polyphase power converter of FIG. 1 can operate in at least three stages of operation. In a first stage of operation, when the AC source 100 is unavailable, the SCRs (e.g., SCRs 105A, 105B) may be operated to provide a constant level of DC power from the DC source of power 101 to the DC inputs of circuits 115A, 115B, 117A, 117B, 119A, 119B. In a second stage of operation, where the polyphase AC source 100 becomes available the power supplied from the DC source 101 can be gradually decreased (e.g., ramped down) while the power supplied from the polyphase AC source 100 is gradually increased. Here, in the second stage of operation, the SCR (e.g., SCR 105A) only conducts for the period of a DC pulse provided from the DC source of power 101 to the inductor (e.g., inductor 116A). In one embodiment, power is supplied from the polyphase AC source 100 via rectifier (e.g., rectifier 103A) during the period when the SCR (e.g., SCR 105A) is not conducting, i.e., when the SCR is turned off.
In a version of the preceding embodiment, each boost converter circuit draws a sinusoidal half-cycle of current from a line of the polyphase AC source 100 to which it is connected during 180 degrees (i.e., half) of the line cycle. DC power is drawn from the DC source of power 101 during a part of the remaining 180 degrees of the line cycle. Generally, DC power is drawn during less than the full 180 degrees. In one embodiment, DC power is drawn from the DC source of power 101 for 120 degrees of the line cycle. In this embodiment, the total current drawn from either the positive DC source or the negative DC source approaches a constant DC current thereby minimizing battery ripple current.
Provided an overload condition does not exist, in a third stage of operation, the power supplied from the DC source 101 is zero and the electrical demand of any load connected to the DC output 115 is met by power supplied to the boost circuits 115A, 115B, 117A, 117B, 119A, 119B from the polyphase AC source 100. Further, power may also be provided from the DC source 101 to circuits 115A, 115B, 117A, 117B, 119A, 119B when the AC source 100 is available to supplement the AC source, for example, during periods of heavy electrical loading. That is, power from the DC source of power 101 can be used to prevent an overload of the polyphase AC source 100.
Because the rectifier included in each boost circuit is only conducting for one half of the line cycle where each boost circuit is connected to a single line of the polyphase AC source 100, each boost circuit can only draw power from the polyphase AC source 100 for a maximum of 180 degrees of the line cycle.